Surface plasmon resonance (SPR) is a conventional method to measure binding stoichiometry, kinetics, and chemical affinity. The principle of SPR is based on the interaction between light and conductive electrons at a surface of a metal film including silver, gold, aluminum or copper. When a resonant condition is fulfilled and surface plasmon is generated on the surface of the metal film, the detected light shows a peak at the resonance wavelength (in spectral interrogation) or at the resonance angle (in angular interrogation). This resonance peak will shift according to the reflective index variation on the surface of the metal film caused by the analyte binding. Thus, it gives the real-time information of the binding events. The kinetics, the affinities and the concentrations of the analyte can be calculated as the dynamic process saturates with time.
However, conventional SPR sensors require apparatuses such as a prism to adjust the momentum of the light so as to match with the momentum of the electrons in the metal film to induce the plasmonic resonance. These conventional sensors are generally very bulky. It is difficult to develop such conventional sensors into a point-of-care sensing system.
Localized surface plasmon resonance (LSPR), which utilizes the interaction between the light and the metal nanostructures, has also been used. LSPR has similar functions as SPR, and LSPR can be generated by directly illuminating the light at any angle onto a metal nanostructure with a dielectric interface. Thus, a portable LSPR biosensing system can be realized. However, it is difficult for LSPR sensors to detect a small molecule (in a nanometer range), as the small molecules cause very little peak shift.
FIG. 1a shows a graph 100 of optical power absorption against wavelength for prostate cancer biomarker prostate-specific antigen (PSA) in blood. Graph 100 shows an absorption spectrum of a gold nanohole array (without the PSA), and an absorption spectrum of the gold nanohole array with the PSA. The gold nanohole array has a thickness of about 100 nm and a pitch of about 400 nm. The diameter of each nanohole of the array is about 150 nm. The diameter of the PSA is about 1 nm.
FIGS. 1b and 1c show magnifications of portion 102 and portion 104 of graph 100. FIG. 1b shows a portion 106 of the absorption spectrum of the gold nanohole array (without the PSA), and a portion 108 of the absorption spectrum of the gold nanohole array with the PSA. FIG. 1c shows a portion 110 of the absorption spectrum of the gold nanohole array (without the PSA), and a portion 112 of the absorption spectrum of the gold nanohole array with the PSA. It can be observed from both FIGS. 1b and 1c that the PSA only causes a negligible resonant change in the absorption spectrum of the gold nanohole array, which needs a highly accurate spectrometer to discern the difference.